Ultraviolet (UV) lamp systems have been in use for many years in applications such as UV curing. Many conventional industrial high wattage electrodeless UV curing lamp systems have been based around a modular 10 inch long lamp (irradiator module) which contains a tubular electrodeless bulb envelope powered by two magnetrons mounted on waveguides that direct microwave energy (RF) into a common elliptical reflector cavity. These modular irradiators can be placed end to end to provide a continuous UV curing zone for applications of variable length. U.S. Pat. No. 4,042,850 illustrates details of such conventional systems, and is hereby incorporated by reference herein in its entirety.
Magnetrons emit microwaves from an output antenna into a metal waveguide inside the lamp which directs the RF into a reflector cavity containing the bulb. The electrodeless, tubular quartz bulb is filled with a small amount of mercury, an easily ionized gas such as Argon along with other additives such as metal halide salts to modify and enhance the spectral output. The high-frequency (e.g., 2.45 GHz), high power (e.g., up to 3 kW RF) electric field generated by each of the two magnetrons excites the gas inside the bulb to high energy levels, vaporizing and ionizing the mercury and additives with an average power up to 600 watts per linear inch. The resulting high energy collisions of the vaporized molecules cause the bulb to emit a large amount of UV energy that also includes some visible and infrared radiation. The enhanced UV output possible with certain bulb fill additives, such as the Iron Iodide additive, is one of the primary advantages of electrodeless UV lamps.
Power for each irradiator module comes from a dedicated power supply connected by cables to that irradiator module. The power supply must include voltage conversion components in order to drive the two magnetrons at high DC voltage. This normally involves a pair of voltage conversion engines with each individual engine connected to a single magnetron. The power supply enclosure also contains many other components for additional services such as user and machine power level control and display interfaces, magnetron filament control logic, protective interlock functions, etc.
The voltage conversion components can be based on a ferro-resonant principle that uses transformers, diodes and capacitors to rectify the AC line voltage and transform the output to the much higher DC voltage utilized by the magnetrons. They can also be based on more modern and efficient solid state high frequency switching components.
Both ferro-resonant and switch mode power supplies are currently available in the market and both are being used interchangeably to drive ten inch microwave lamp irradiators such as described above. Each of the two power supply designs have advantages and disadvantages.
Microwave powered lamp systems using ferro-resonant power supplies generally have high ripple magnetron current characteristics which therefore generate high ripple UV output. A transformer for the second magnetron is generally powered with a different leg (different from the leg for the first magnetron) of a 3 phase supply line, and therefore the ripple of the second magnetron can be 120° out of phase with the first magnetron. Ferro-resonant power supplies tend to be less efficient compared to solid state power supplies. Ferro-resonant power supplies are very heavy due to the heavy transformers (e.g., up to 4 times heavier). Slot arcing and magnetron internal arcing or moding tends to be a more significant problem when using high ripple power supplies due to the extra high peak current and voltage that occur compared to low ripple solid state power supplies. Ferro-resonant power supplies must have separate versions for the different line voltages and frequencies (50 or 60 Hz) used worldwide. The magnetron RF output power can also drop to near zero 100-120 times a second and may peak at levels much higher than the average power. Likewise, the UV output from either end of the bulb closely follows the modulation ripple of the respective magnetron current. Normally the power supply is wired with 3 phase AC power such that one magnetron can operate out of phase from the second magnetron. In this case the mixing of the UV in the focal plane of the lamp will mitigate some of the high ripple effect. However, the ripple at the focal plane is most evident towards the ends of the lamp module since there is less contribution from the far end of the lamp. This low point in the modulated UV output can be a problem for high speed curing applications passing under the lamp since all areas of the coating to be cured may not be uniformly cured. In wide web applications, using multiple lamp modules stacked end to end, there can be areas cured less than other areas due to the same high ripple issue especially when the two adjoining magnetrons happen to be operating in-phase.
Modern solid state DC switching power supplies designed for electrodeless UV lamps have been available for many years with enhanced features such as more precise power level control. With switching frequencies above the KHz range they provide a virtually continuous UV output with no ripple. They also do not suffer from the disadvantages noted above for ferro-resonant power supplies.
However, a major problem when operating with continuous, no ripple, output power supplies has commonly been referred to as “color separation” along the axis of the bulb when operating additive bulb types vertically. Many curing applications (e.g., certain rotating bottle or cans applications, certain optical fiber draw tower applications, etc.) tend to utilize a vertical lamp orientation. High performance additive bulbs such as the “D” bulb type (e.g., having an Iron Iodide additive) and the “V” bulb type (e.g., having an Gallium Iodide additive) are both subject to color separation when vertical and powered by a low ripple supply. The “D” type bulbs are widely used due to superior enhancement of the UVA output, and the “V” bulb type is enhanced in the UVV band. However, they are not able to maintain an adequate distribution of the fill material inside the bulb when the bulb axis is angled or used in vertical operation. Both of these bulbs, and other additive bulbs, perform poorly and become inefficient when operated vertically with DC power.
When a conventional bulb (e.g., a 9 inch long cylinder of varying diameter) is operated in a non-horizontal (e.g., vertical, angled, etc.) geometry, the bulb fill and additives tend to migrate more to the bottom of the bulb resulting in an undesirable operating condition. Microwave coupling to the upper half of the bulb is greatly reduced due to a lack of adequate vaporized fill materials remaining and the overall UV output is diminished by about 25%. In addition the bulb tends to run significantly hotter than normal in the lower half of the bulb. For example, excessively high peak bulb wall temperatures (e.g., greater than 1150° C.) may result from non-uniform wall loading, and may reduce bulb life considerably and may result in catastrophic bulb envelope failures due to severely overheated bulb envelopes.
Thus, it would be desirable to provide systems and methods for improving operation of UV lamps especially during operation of the lamp in a non-horizontal (e.g., vertical, angled, etc.) orientation.